Numerous devices are available for providing accurate linear measurements. Height gauges are available to provide the height of an article. Dial gauges, Digamatic.TM. indicators, and linear gauge systems are used to precisely measure the thickness or height of an article. Linear and X-Y location and displacement measuring systems are available to accurately measure the linear displacement or the exact position of an article.
Many of the linear measurement systems in use today include a housing having a measuring scale slidably mounted in the housing. A driving member, such as a spindle, is coupled to the scale and to the article whose displacement is being measured. A bushing mounted on the housing and surrounding the spindle is typically used to ensure that the spindle slides smoothly in the housing. A sensor within the housing senses movement of the scale and provides an output indicative of the position of the article, movement of the article, or both. In the prior art, typically, for precision application, this sensor is spaced at a specific distance from the scale, and expensive precision bearings are employed to avoid wear and provide accurate guiding and spacing of the elements.
Different types of sensing systems, scales, and suspension systems are known in the prior art. U.S. Pat. No. 4,603,408, to Sakagami describes a linear scale having an optical grating and an optical sensor for determining movement of the scale. In this type of device, the sensor is typically guided along a stationary scale by roller bearings. A capacitive-type position sensor which senses changes in capacitance to measure displacement is described in U.S. patent application Ser. No. 07/372,773, entitled "Capacitive-Type Measuring Device for Absolute Measurement of Positions" and commonly assigned now U.S. Pat. No. 5,023,559. No particular suspension system is specified, but electronic calipers which are now commonly available exhibit a typical guiding system for this type of device. Such a guiding system is not suitable for high precision measurements. United Kingdom Patent No. 1,366,284 also describes a capacitive sensor in a length measuring system. In many environments, a capacitive sensor is preferred; they are not as complex, fragile, or expensive as optical sensors.
One of the problems with sensors which measure the change in parallel plate capacitance to determine movement of a plate along an axis in the plane of the plate is that the capacitance varies based on movement perpendicular to the plate as well as axial movement along the measuring axis parallel to the plate. As the scale undergoes movement along its measuring axis, the change in capacitance caused by the scale is sensed and electronically processed to determine displacement of the scale. Unfortunately, any movement perpendicular to the axis can cause a change in capacitance which is not caused by axial movement. This undesirable change in capacitance creates errors in sensing the movement of the measured object. As a result, known capacitive length-measuring systems are susceptible to errors unless relatively expensive bearing systems are employed. Another important practical problem with sensors which measure the change in parallel plate capacitance to determine movement of a plate in the plane of the plate is that the capacitive signal strength diminishes rapidly with decreasing plate dimensions and increasing gap, while the costs of practical capacitive encoders decrease approximately with the encoder area (a close analogy can be drawn to the cost behavior of silicon IC's). A small capacitive plate area is a highly desirable practical feature. However, in order to reduce the capacitive plate area while still maintaining sufficient signal strength, a smaller capacitive gap must be maintained. Unfortunately, according to previous suspension designs, a small gap exaggerates the aforementioned sensitivity to non-axial (error-producing) motions and increases the chances of damage to the sensor due to inadvertent contact between the closely spaced relatively moving pieces. In the prior art, expensive precision bearing systems or tedious close-tolerance assembly techniques have been employed to achieve a small operating gap. Either method substantially offsets the cost and versatility benefits of reduced sensor size. Even so, use of an optical measuring system is not permitted in some environments and, where permitted, is significantly more expensive and more delicate such that use of the optical system may not be permitted under some circumstances.